GB2394192A

GB2394192A - Urea solution storage for exhaust gas treatment

Info

Publication number: GB2394192A
Application number: GB0318804A
Authority: GB
Inventors: Johannes Schaller; Thomas Brinz; Ilona Ullmann
Original assignee: Robert Bosch GmbH
Current assignee: Robert Bosch GmbH
Priority date: 2002-08-14
Filing date: 2003-08-11
Publication date: 2004-04-21
Anticipated expiration: 2023-08-11
Also published as: DE10237165A1; US7100367B2; FR2843612B1; DE10237165B4; GB0318804D0; FR2843612A1; GB2394192B; US20040103652A1

Abstract

A device for treating exhaust gas comprises a storage unit for urea/water solution. The solution comprises at least one additive, the additive chemically influencing said solution. The additive may also be an anti-freezing agent, and may be ammonia. A production device for producing the additive may be provided. A heating device for warming up the urea/water solution may be provided. The storage unit and the production device may be designed as a single unit. The additive may be designed to chemically stabilise the urea/water solution, and may be carbon dioxide. A second storage unit may be provided for storing the additive, and a pressure generating unit may be provided to pressurise the additive. The second storage unit may be designed as a pressure storage unit for pressurised storage of the addition substance. A pressure sensor may be provided to measure storage pressure.

Description

"Device for treating the exhaust gas from a combustion apparatus" 5 The

invention relates to a device for treating the exhaust gas from a combustion apparatus, in particular a diesel combustion apparatus, in accordance with the pre-

characterising clause of claim 1.

10 For the after-treatment of exhaust gases, in particular diesel exhaust gases from diesel vehicles, the so-called "selective catalytic reduction method" (SCR method) has hitherto been employed for the purpose of removing nitrogen oxides (NOX). In this method, the reducing agent I 15 ammonia (NH3), which is selectively converted with nitrogen oxides, inter alla, to form molecular nitrogen, is produced in the vehicle in suitable reactors.

In the most common SCR method in the vehicle sector, a 20 urea/water solution (UWS) is injected into the exhaust-gas tract, said UWS decomposing under urea hydrolysis to form NH3 and carbon dioxide (CO:). In order to assist the hydrolysis, a so-called "hydrolysis catalyzer" is usually located downstream of the injection point in the direction 25 of flow of the exhaust gas.

For the purposes of the invention, a "urea/water solution" (UWS) is understood to mean a liquid or solution which comprises at least urea and water. In the 32.5% solution 30 normally used, however, the low resistance to freezing (eutectic at -11 C) is particularly disadvantageous, especially in vehicle applications.

What is also disadvantageous about the use of UWS is the 35 fact that it decomposes as a result of elevated temperatures from about 60OC to 80OC onwards. Under these

circumstances, ammonia (NH3) and carbon dioxide (COP) is produced from urea. The consequence of this is that, because of this thermolysis, the solution, which is often originally a 32.5 percent one, has a lower urea 5 concentration which is generally unknown. Without precise knowledge of the urea concentration, however, optirnised denitrification of the exhaust gases cannot be guaranteed since, on the one hand, toxic ammonia is emitted if the addition of urea is too large or, on the other hand, a 10 relatively large amount of NOx is emitted if the addition of urea is too small.

There is currently already a discussion about direct or indirect detection of the changing concentration of urea in 15 the UWS. However, suitable methods of measurement are I realizable only with a comparatively high outlay in economic and constructional terms.

In contrast to this, the object of the invention is to 20 propose a device for treating the exhaust gas from a combustion apparatus, in particular a diesel combustion apparatus, said device having a first storage unit for storing a urea/water solution and it being possible to realise optimised treatment of the exhaust gas without 25 great outlay.

Starting out from a device of the type initially mentioned, this object is achieved through the characterizing features of claim 1.

Advantageous embodiments and further developments of the invention are possible by means of the measures mentioned in the subclaims.

35 Accordingly, a device according to the invention is distinguished by the fact that the urea/water solution

! comprises at least one additive for chemically influencing the urea/water solution.

The additive is preferably designed as an operating 5 substance for chemically stabilising the urea/water solution. At least one metering device for metering-in the addition substance or operating substance Is advantageously provided. By means of an operating substance of this type, which is to be introduced into the first storage unit by 10 means of the metering device according to the invention, the decomposition of the UWS can be prevented to a very great extent. What is hereby achieved, according to the invention, is that the solution, which is generally known and is, for example, a standard 32.5 percent one, does not 15 significantly decompose and thus a very largely accurate metering-in of the quantity of urea relevant to the SCR into the exhaust-gas tract of the combustion apparatus of a vehicle, for example a lorry or motor car, is guaranteed.

By this means it is possible, in particular, to almost 20 prevent the socalled "ammonia slip".

In an alternative variant of the invention, in particular, the urea/water solution comprises, as additive, at least one anti-freezing agent for lowering the freezing point of 25 the urea/water solution.

Freezing-up of the urea/water solution, even at fairly low temperatures, is effectively prevented in a particularly neat way with the aid of an anti-freezing agent of this 30 type. This permits operation of the device according to the invention without breakdowns, even at winter temperatures in the temperate or polar climatic zones.

This is a decisive advantage over the prior art,

particularly in vehicle applications.

In one particular further development of the invention, the additive or anti-freezing agent at least comprises ammonia

( (NH:), or else the anti-freezing agent essentially consists of ammonia (NH3). Anti-freezing agents which are already known in the vehicle sector, such as ethylene glycol for example, considerably impair the SCR process or lead to 5 disadvantageous exhaust-gas emissions, something which is completely contrary to the object of the exhaust-gas treatment, and are also comparatively expensive in economic terms. In addition, the massrelated reducing agent content in the UWS is decreased as a result of dilution 10 with suitable anti-freezing agents.

By means of an ammonia-containing anti-freezing agent or with the aid of almost pure ammonia, on the other hand, the freezing point of the UWS is lowered, and the mass-related! 15 reducing agent content of the UWS markedly increased, in an economically favourable manner. Furthermore, ammonia has very good solubility in water and in an aqueous solution of urea. 20 In an SCR method having a device according to the pre-

characterising clause of claim 1, an at least ternary mixture of ammonia, urea and water, optionally with other addition substances such as emulsifying agents, stabilising agents, etc., is generally used, according to the 25 invention, as the reducing agent instead of the binary mixture of urea and water which has hitherto been usual.

A ternary mixture of ammonia, urea and water may optionally be readymanufactured beforehand and, in particular, be 30 held in a tank on a vehicle or introduced into the storage unit of the device according to the invention. A suitably ready-mixed, ternary mixture may possibly be used and/or produced according to the invention in a seasonal manner, that is to say preferably during the winter phase.

As an alternative to this, or in combination therewith, at least one suitable metering device for metering the

( additive or anti-freezing a-tent into the urea/water solution is provided in advantageous manner. At least one production unit for producing the anti-freezing agent is preferably provided, which production unit is designed, i 5 particular, as a metering device. As a result of this, the anti-freezinq agent may be produced, for example by conversion of an operating substance, that may possibly already be present, for the device according to the invention and/or for the vehicle, so that it is possible to 10 dispense with separate tanking-up, or the like, of the anti-freezing agent. This can markedly decrease the outlay for operating the device according to the invention.

In one advantageous form of embodiment of the invention, 15 the production unit has at least one heating device for warming up the urea/water solution. By means of this measure, it is possible to produce ammonia by thermolysis of the urea/water solution. The heating device is operated electrically, for example, and/or may be realised in the 20 form of a heat-exchanger for making use of waste heat from a heat-generating component, in particular one belonging to the vehicle. The waste heat from the combustion apparatus may possibly be used, optionally with the aid of a heating fluid, for operating the heating device.

The production unit advantageously comprises at least one catalytically active converting element for the catalytic conversion of the urea/water solution. A production unit designed as a catalyzer improves the production of ammonia 30 in an advantageous manner. The converting element may optionally be designed as a catalytically active coating which preferably has contoured portions that enlarge the surface. This additionally improves the conversion of the urea/water solution or the production of the ammonia.

In one particular further development of the invention, the storage unit is designed as a production unit. For

example, a predetermined region of the storage unit may be warmed up by means of a heating device and the contact face between the storage unit and the UWS may optionally be catalytically coated. As an alternative to this, a 5 structurally separable production unit may be disposed inside the storage unit, or be integrated into the latter.

It is also possible for the production unit to be disposed outside the storage unit and to be connected to the latter 10 by means of suitable lines.

It is basically possible, with an almost constant, mass-

related reducing agent content, to decrease the urea fraction in the UWS and to adapt the ammonia fraction! 15 accordingly. This is realised, in particular, in those variants of the invention in which ammonia is produced by means of the OWS held in a tank or stored in the storage unit. In these cases, according to the invention, there is both a lowering of the freezing point and, in addition, the 20 load on the so-called "hydrolysis catalyzer" normally used, which is disposed in the exhaustgas tract, downstream of the point of injection in the direction of flow of the exhaust gas, is relieved. The said "hydrolysis catalyzer" decomposes the urea in the UWS to form ammonia and carbon 25 dioxide, so that the so-called "SCR catalyzer" which is often separate and is disposed in the exhaust-gas tract, downstream of the "hydrolysis catalyzer" in the direction of flow, selectively denitrifies the exhaust gas in known manner. In the aforesaid variant, moreover, because of the decreased quantity of urea or decreased urea fraction, the risk of clogging-up of the feeding-in system that conveys the urea/water solution or the urea/water/ammonia solution 35 into the exhaust-gas tract, particularly by crystallised-

out urea, is markedly reduced.

( In another variant of the invention, the mass-related reducing agent content is decisively increased, with a constant urea content in the OWS, by ammonia which is additionally introduced, the freezing point of the solution 5 likewise being lowered in accordance with the invention.

The additive or operating substance advantageously at least comprises carbon dioxide, or the operating substance essentially consists of carbon dioxide. What is achieved 10 with the aid of one of these measures is that the chemical balance of the UWS decomposition is shifted onto the urea side because of the CO2 feed, so that the decomposition of the UWS to form ammonia and carbon dioxide is prevented in a particularly simple manner. I The formation of gaseous ammonia in the first storage unit is avoided, to a very great extent, by the metering-in of carbon dioxide, so that the reducing agent concentration in the UWS is not decreased.

In addition, in one particular variant of the invention, the introduction of gaseous ammonia from the tank unit or

first storage unit into the SCR catalyzer may optionally be dispensed with since, in contrast to the prior art, almost

25 no gaseous, toxic ammonia is formed. As a result of this, the outlay for the exhaust-gas treatment or denitrification of suitable exhaust gases is decreased according to the invention. 30 The addition of one of the reaction products, in particular, into the first storage unit is generally advantageous, since the chemical balance of the reaction is shifted onto the educt or educts side, that is to say, in this case, onto the urea side. Consequently, the addition 35 of ammonia by means of the metering device is also conceivably possible according to the invention.

! In one particular further development of the invention, at least one second storage unit for storing the additive or operating substance is provided. This permits the possibility, in particular, of metering in the operating 5 substance or the carbon dioxide in dependence upon the filling level or the quantity of UWS in the first storage unit. The first storage unit and/or the second storage unit may 10 optionally be under atmospheric pressure conditions. At least one pressure-generating unit is preferably provided for pressurizing the additive or operating substance. With the aid of this measure, the operating substance may, for example, be transported out of the second storage unit into ill 15 the first storage unit.

Furthermore, it is possible, by means of the pressure-

generating unit, to generate an overpressure, that is to say a pressure greater than atmospheric pressure, in the 20 first storage unit, which can lead to a further shift in the chemical balance towards the educt side. This brings about additional stabilization of the urea concentration in the UWS.

25 The pressure-generating unit preferably has at least one regulating unit for setting the pressure. With the aid of a suitable regulating unit, it is possible, for example, to set an almost constant pressure, irrespective of the filling level of the UWS, above all in the first storage 30 unit.

In one advantageous variant of the invention, the second storage unit is designed as a pressure storage unit for the pressurized storage of the operating substance. With the 35 aid of this measure, the second storage unit may be simultaneously used as a pressure-generating unit according to the invention. This leads to the second storage unit,

( which is constructed as a pressure storage unit, transporting the additive or operating substance to the first storage unit by means of a pressure difference or pressure drop.

s Furthermore, an increase in the density of the stored additive or operating substance is realised by means of a second storage unit designed as a pressure storage unit, so that said second storage unit is capable, with a relatively 10 small volume, of storing a comparatively large quantity of additive or operating substance. The pressure storage unit may, for example, be designed as a CO2 cartridge, compressed CO2 gas cylinder or the like.

15 The metering device preferably comprises at least one metering valve. This permits the possibility of metering the additive or operating substance into the first storage unit in a comparatively simple manner. Generally speaking, the metering valve is advantageously controlled or 20 regulated by means of the regulating unit or the like.

In one preferred form of embodiment of the invention, at least one pressure sensor is provided for ascertaining the storage pressure of, possibly, the first and/or second 25 storage unit. A pressure sensor improves monitoring of the device, particularly by means of the regulating unit, in an advantageous manner. For example, it is possible, by means of a pressure sensor, to ascertain the degree of filling of the pressure storage unit or second storage unit. By means 30 of a pressure sensor, moreover, it is possible to ascertain the pressure of the first storage unit and to set a maximum and/or minimum pressure in the first storage unit, optionally with the aid of an advantageous aerating or ventilating device, for example by means of an aerating or 35 ventilating valve. An overpressure of about 100 millibars and/or an underpressure of about 20 millibars may optionally be predetermined, so that the first storage unit

( should, in general, not be constructed as a pressure vessel or the like. This leads to a realization of the first storage unit, for example as a plastic tank or the like, which is favourable in constructional and economic terms.

Basically, it is conceivably possible to provide, for example, an intermediate-storage unit for the intermediate storage of gases from the first storage unit. As a result of this, gas which is, above all, CO2containing and may 10 optionally be charged with traces of ammonia, is discharged for the purpose of reducing pressure, for example in the first storage unit. This gas mixture may possibly be fed to the second storage unit for re-use and/or, preferably during operation, to the combustion apparatus.

( An exemplified embodiment of the invention is represented in the drawings and will be explained in greater detail 5 below with the aid of the figures.

In detail: figure 1 shows, in a diagrammatic representation, a 10 device according to the invention with an externally disposed reactor; figure 2 shows, in a diagrammatic representation, another device according to the invention 15 with an integrated reactor; and figure 3 shows, in a diagrammatic representation, another device according to the invention with a second storage unit.

A device according to the invention for treating the exhaust gas from a combustion unit, of which no further details are represented in figures 1 or 2, comprises a tank I which stores, in particular, a urea/water solution 25 (UWS) 2. The UWS 2 is fed, by means of a discharge line 3 disposed in the lower region of the tank 1, to a feeding system, of which no further details are represented, for feeding the stored operating liquid to the exhaust-gas tract of, for example, a vehicle with a diesel engine. The 30 device for exhaust-gas treatment according to the invention is often installed in a commercial vehicle or motor car.

However, its use in a stationary combined heat-and-power plant or the like is also conceivably possible. For the SCR-type reduction of the exhaust gases, the exhaust-gas 35 tract may, for example, comprise a hydrolysis catalyzer as well as an SCR catalyzer in known manner.

( According to figure 1, a reactor 4 is disposed outside the OWS tank 1. The reactor 4 is connected to the line 3 in such a way that it receives part of the flow of fluid flowing out of the tank 1 and preferably converts, above 5 all, the urea of the UWS 2 into ammonia, in a manner of which no further details are represented, with the aid of a heating device and a catalytically active coating. As a result of this, a significantly ammonia-enriched UWS 2, or a ternary mixture of urea, water and ammonia, flows back 10 into the tank 1 from the reactor 4 by means of a line 5.

In this way, conduction of the UWS 2 in a circuit is realised. These measures lead to a gradual increase in the ammonia fraction in the UWS 2 and to a lowering of the freezing point of the OWS 2 stored in the tank 1 or of the 15 ammonia-enriched UWS 2.

According to figure 2, the reactor 4, which produces ammonia by means of a heat supply Q and an advantageous, catalytically active coating, is integrated into the 20 tank 1. In this instance, the UWS 2 flows through the reactor 4, ammonia (NH3) or a markedly ammonia-enriched UWS 2 or a ternary mixture of ammonia, urea and water being produced and stored in the tank 1. In this variant of the invention, too, the freezing point of the UWS 2 stored in 25 the tank 1 is lowered in an advantageous manner because of the dissolved ammonia (NH3).

Furthermore, other anti-freezing agents, possibly in addition to the ammonia, may also be added to the OWS 2 30 stored in the tank 1.

Figure 3 shows, in a diagrammatic representation, a device according to the invention for treating the exhaust gas from a combustion apparatus of which no further details are 35 represented. The device according to the invention comprises a UWS tank 1 which stores a urea/water solution (UWS) 2. The UWS 2 is fed to a urea-metering system, of

( which no further details are represented, by means of a discharge line 3 disposed in the lower region of the tank 1. The urea-metering system feeds the UWS 2 to the exhaust-gas flow of the combustion apparatus, for example 5 the diesel motor. For the SCR-type reduction of the exhaust gases, the exhaust-gas tract may comprise, for example, a hydrolysis catalyzer as well as an SCR catalyzer in known manner.

10 For the purpose of stabilizing the UWS 2 in the tank 1 according to figure 3, carbon dioxide (CO2) 15 is metered into said tank by means of a CO2 pressure cartridge 14. A shut-off valve 16 is disposed on the appropriate line between the CO2 cartridge 14 and the tank 1 for the purpose 15 of regulating the quantity of CO2 to be metered in.

Moreover, the UWS tank 1 according to figure 3 has an overpressure valve 7 which opens, in particular, at an overpressure of about 100 millibars and may optionally 20 discharge excess CO2 15. The shut-off valve 16 guarantees metering-in of the CO2 15 from the cartridge 14, it being necessary to select the metering pressure to be lower than a maximum pressure set by means of the overpressure valve 7.

A pressure sensor, of which no further details are represented in figure 3, may optionally be provided for monitoring the pressure in the tank 1 and, optionally, for monitoring or ascertaining the degree of filling of the 30 cartridge 14. The pressure sensor of the tank 1 is generally used, inter alla, for regulating the shut-off valve 16.

Basically, the devices in figures 1, 2 or 3 may, according 35 to the invention, be combined with one another in an unrestricted manner.

Claims

Claims:

1. Device for treating the exhaust gas from a combustion apparatus, in particular a diesel combustion apparatus, 5 which device has a storage unit (1) for storing at least one urea/water solution (2), characterised in that the l urea/water solution (2) comprises at least one additive for chemically influencing said urea/water solution (2).

10

2. Device according to claim. 1, characterised in that the additive is designed at least as an anti-freezing agent for lowering the freezing point of the urea/water solution (2).

3. Device according to one of the aforementioned claims, 15 characterised in that the anti-freezing agent comprises at least ammonia (NH).

4. Device according to one of the aforementioned claims, characterised in that at least one metering device (4) for 20 metering the additive into the urea/water solution (2) is provided.

5. Device according to one of the aforementioned claims, characterised in that at least one production unit (4) for 25 producing the additive is provided.

6. Device according to one of the aforementioned claims, characterised in that the production unit (4) has at least one heating device for warmingup the urea/water 30 solution (2).

7. Device according to one of the aforementioned claims, characterised in that the production unit (4) comprises at least one catalytically active converting element for the 35 catalytic conversion of the urea/water solution (2).

l5

8. Device according to one of the aforementioned claims, characterised in that the storage unit (1) is designed as the production unit (4).

5

9. Device according to one of the aforementioned claims, characterised in that the metering device (4) comprises at least the production unit (4) .

10. Device according to one of the aforementioned claims, 10 Gharacterised in that the additive is designed at least as an operating substance (15) for chemically stabilizing the water/urea solution (2).

11. Device according to claim 1, characterised in that the I l5 operating substance (15) comprises at least carbon l dioxide (15).

12. Device according to one of the aforementioned claims, characterised in that at least one second storage unit (14) 20 for storing the addition substance (15) is provided.

13. Device according to one of the aforementioned claims, characterised in that at least one pressure-qenerating unit (14) for pressurising the addition substance (15) is 25 provided.

14. Device according to one of the aforement.ir-:ned claims, characterised in that the second storage unit (14) is designed as a pressure storage unit (14) for the 30 pressurized storage of the addition substance (15).

15. Device according to one of the aforementioned claims, characterised in that at least one pressure sensor for ascertaining the storage pressure is provided.

16. Vehicle, in particular a motor car, having a device for treating the exhaust gas from a combustion apparatus, in

( particular a diesel combustion apparatus, wherein a storage unit (1) for storing at least one urea/water solution (2) is provided, characterized in that the device is designed in accordance with one of the aforesaid claims.

17. Method of treating the exhaust gas from a combustion apparatus, in particular a diesel combustion apparatus, which method has a storage unit (1) for storing at least one urea/water solution (2), characterized in that use is 10 made of at least one urea/water solution (2) having an additive for chemically influencing said urea/water solution (2).

18. Method according to claim 17, characterised in that use I is made, as the additive, of an anti-freezing agent for lowering the freezing point of the urea/water solution (2).

19. Method according to one of the aforesaid claims, characterized in that use is made of a device according to 20 one of the aforesaid claims.

20. A device for treating exhaust gas substantially as herein described with reference to the accompanying drawings.

21. A vehicle substantially as herein described with reference to the accompanying drawings.

22. A method of treating exhaust gas substantially as 30 herein described with reference to the accompanying drawings.